Micro-optic lenses, often called microlenses, are, generally speaking, small, short focal length lenses which have numerous uses in the opto-electronics field. The term "microlens" covers both the 30 .mu.m microscopic lenses disclosed in U.S. Pat. No. 4,689,291 as well as larger but still small, such as 4 mm (4.times.10.sup.3 .mu.m) diameter, lenses which may be manufactured by apparatus and methods in accordance with the present invention. Applications for such lenses include optical couplings for fiber optics, collecting the light emitted from an LED or laser diode, focusing light on solid state photodetectors, use in integrated optical couplers/decouplers, and objective lenses for optical disk and CD players.
These and other miniaturized opto-electronic devices have created a demand for microlenses and microlens arrays, and substantial effort and expense have been devoted to the development of such lenses. Depending on their size, microlenses and microlens arrays can be molded by replicating the contours of precisely machined dies.
Microlenses can also be formed by producing a gradient index lenslet in a substrate, which may be of glass, plastic, organic/inorganic composition or sol-gel. Localized index changes are produced by diffusion of mono valent ions (Ag+, Tl+), or low molecular weight monomers, or by ion implantation. In all these processes, a metal mask having small circular openings is first placed over the chosen substrate by a process of metalizing one substrate surface, then using photolithography to make an array of openings with equal spacing between centers. Ion exchange through the small circular openings changes the localized index of refraction via binary diffusion to build an index distribution with iso-index contours that are spherical in shape (i.e. lens shaped). The index change is largest at the opening and decreases radially outward to that of the substrate. Ion exchange or diffusion can be also made in a sol-gel or organic/inorganic composite substrate. By a second method, the exchange between a monomer and partially polymerized polymer host substrate of differing refractive index can build a micro lens array. Final thermal polymerization gives a rigid array of lenslets. In a third method, high energy ion implantation, through circular openings in a metalized mask, of such elements as Pb+, Au+, etc., followed by thermal diffusion, can give the requisite index change and lens shaped contour. Through thermal treatment of the ion implanted substrate, atomic diffusion develops a spherical lens shape which extends under the mask opening. It is claimed that microlenses can be made aspherical shaped through an electrical bias on the substrate during the process of forming such microlenses, to enhance their focal properties. The effects of such electrical bias are believed to be rather limited.
U.S. Pat. No. 4,514,053 discloses an integral optical device that is composed of a photosensitive glass having an optical pattern developed therein by a refractive index change due to formation of colloidal metal particles and/or crystalline microphases nucleated by such particles. In a specific embodiment the pattern is composed of at least one transparent lens system having a radial gradient refractive index distribution.
U.S. Pat. No. 4,689,291, discloses the formation of non-gradient microlenses and microlens arrays on opto-electronic devices and other substrates by using "sharp" (approximately 90.degree.) edge pedestals to confine the lateral flow of molten lens material. The process for the production of such microlenses includes the steps of: (a) depositing a thin, optically opaque material, such as an aluminum film, on the desired substrate, such as quartz; (b) photolithographically patterning the aluminum film with an array of precisely dimensioned apertures, which form stops for the microlenses; (c) forming circular sharp edged pedestals, also using photolithography techniques, on top of the aperture stops formed in the aluminum film; (d) forming, on top of the pedestals and also by conventional photolithography techniques, cylinders of photoresist; and (e) melting the photoresist to form the lenslets. The cylinder of photoresist is smaller in diameter than the diameter of the pedestal and positioned on the pedestal so that there is adequate clearance between the respective outer circumferences to prevent the formation of unwanted drip paths that would allow the photoresist to spill over the pedestal edges. During the melting step the molten photoresist wets the pedestals, while the pedestals remain hard, so that the photoresist spreads laterally thereacross. The "sharp" edges of the pedestals "effectively confine the flow, thereby preventing the molten photoresist from spreading therebeyond." The inherent surface tension of the photoresist, while in its molten state, causes the microlens so formed to have what is stated to be "substantially constant radii". The patent also states that: "Gravitational force may tend to deform the microlenses . . . if they are too large, but no significant gravitational deformation occurs if the internal pressure of the molten photoresist is much greater than the gravitational pressure." Non-hemispherical microlenses may be fabricated by the use of elliptically shaped pedestals. In an article entitled "Technique for monolithic fabrication of microlens arrays", by Z. D. Popovic et. al., Applied Optics, Apr. 1, 1988, the authors disclose an example wherein the resulting lenslets have a diameter of 30 .mu.m and a thickness of 12 .mu.m.
The above described process has several limitations/disadvantages. The microlenses produced are limited to micron size lenslets; the lenslet materials are soft and not scratch resistant; the lenslets are less than hemispherical and, therefore, have long focal lengths (i.e., focal lengths that are many times the thickness of the lenslets); the lenslets produced do not have gradient index profiles and the process does not lend itself to the use of gradient index materials; and the lenslets produced are not aspherical.
My prior U.S. Pat. No. 4,022,855 discloses a method and apparatus for making plastic optical elements having a gradient index of refraction. The method involves introducing first and second monomers, each having a different index of refraction, into a centrifugal mold in accordance with a controlled procedure to produce a lens which has a continuous gradient in refractive index extending in a direction radially outward from the rotational axis of the centrifugal mold. The apparatus includes a means to continuously vary the volumetric ratio of the two monomers as the mold is filled.